Preview changes to controller documentation

[sftim]

This is related to issue #4135 about a concept-level guide to controllers.

This PR isn't here to be merged. Instead, I'm pushing it up as a way to let people see the set of changes I've got on mind all in one place.

Based on the work I've done here, I plan to carve off smaller chunks that are easier to review, and PR those in turn.

For feedback:

• if you have a view on how to tackle Need top-level Controllers concept guide #4135 overall, I recommend commenting there.
• if there is a particular pull request covering part of these changes, I'll reference this PR from the smaller ones, and I recommend giving feedback there
• otherwise, or if it just feels like the right fit, comment on this draft PR.

/hold not-for-merging

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[kbhawkey]

hello @sftim .
This is a good start to the Controller overview. I think this page is fairly important. This page could be treated as a separate PR to kick off the other controller additions/changes. Get tech review for this page.
Here are a few ideas/questions I have about What is a Controller?:

• How does the first sentence tie in to the k8s controller concept (the second sentence)?
• What is a built-in controller? How does a built-in controller differ from a custom controller?
• Are the controllers executors as well as state maintainers?
• What happens if a controller fails? Is the controller restarted?
• The Controller pattern mentions well-implemented, but does not say how to implement one
• Add a What's next section to tie in additional controller resources, such as kubebuilder?
• Clients of controllers?

[k8s-ci-robot]

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[k8s-ci-robot]

@sftim: PR needs rebase.

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[zacharysarah]

@sftim I'd love to see this move forward. Please feel free to /reopen this PR when you're ready!

/close

[k8s-ci-robot]

@zacharysarah: Closed this PR.

In response to this:

@sftim I'd love to see this move forward. Please feel free to /reopen this PR when you're ready!

/close

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.